The present invention relates to novel organic dye compounds, particularly, to trimethine cyanine dyes-which are useful in high density optical recording media.
In a multimedia age, optical recording media such as a compact disc recordable (CD-R, a write-once memory using compact disc); and a digital versatile disc (DVD-R, a write-once memory using digital video disc), are now of great importance. Optical recording media can be classified roughly into inorganic optical recording media which have recording layers composed of inorganic substances such as tellurium, selenium, rhodium, carbon, or carbon sulfide; and organic optical recording media which have recording layers composed of light absorbents that are mainly composed of organic dye compounds.
Among these optical recording media, organic optical recording media are usually prepared by dissolving a polymethine cyanine dye in an organic solvent such as 2,2,3,3-tetrafluoro-1-propanol (abbreviated as xe2x80x9cTFPxe2x80x9d hereinafter), coating the solution onto the surface of a polycarbonate substrate, drying the solution to form a recording layer, and sequentially attaching closely a reflection layer made of a metal such as gold, silver or copper and a protective layer made of an ultraviolet ray hardening resin onto the surface of the recording layer. When compared with inorganic optical recording media, organic ones have the drawback that their recording layers may be easily changed by exposure to light such as reading and natural light, but have the advantage that they can be manufactured at a lower cost because their recording layers can be formed by preparing solutions of light absorbents and directly coating the solutions onto the surface of substrates. Also, organic optical recording media are now becoming the predominant low cost optical recording media because of the advantages that they are mainly composed of organic substances so that they are substantially free of corrosion even when contacted with moisture or sea water; and because information, which is stored in optical recording media in a prescribed format, can be read out using commercialized read-only players using thermal-deformation-type optical recording media, a kind of organic optical recording media.
What is urgently required of organic optical recording media is to increase their recording capacity to suit this multimedia age. The research for such an increment now eagerly continued in this field is to shorten the wavelength of 775-795 nm now used as a writing light to the wavelength of 700 nm or less. However, most of the polymethine cyanine dyes, which had been explored for CD-Rs, could not suitably read and write information using laser beams with wavelengths of 700 nm or less, and therefore they could not fulfill the need for high storage density required in many fields.
There exists a problem of the thermal decomposition of dyes as another causative of hindering the high storage density in organic optical recording media. In organic optical recording media, pits are formed by using the heat, generated when dyes absorb laser beams, to be melted and decomposed, however, most of conventional polymethine cyanine dyes have relatively high decomposition points independently apart from their melting points, and the temperature difference between the melting points and decomposition points; and thus, the formation of pits becomes rough when irradiated with laser beams, and the decomposition heat conducts to around the irradiated points and deforms the already formed adjacent pits, resulting in a difficulty of stably forming minute pits on restricted sites at a relatively high density.
In view of the foregoing, the object of the present invention is to provide organic dye compounds having satisfactory properties in light absorption and thermal resistance.
To attain the above object, the present inventors eagerly studied and screened compounds and found that specific trimethine cyanine dyes (hereinaftermaybe abbreviatedas xe2x80x9ccyanine dyesxe2x80x9d), which are obtainable through a step of reacting a benzoindolium compound having a compatible leaving group with a pyrazinoimidazolium compound having an active methyl group; or reacting a benzoindolium compound having an active methyl group with a pyrazinoimidazolium compound having a compatible leaving group, have absorption maxima in a visible region and substantially absorb visible light with wavelengths around 650 nm when in a thin layer form, and have a relatively high decomposition points and thermal resistance. The present inventors also confirmed that these cyanine dyes stably form minute pits on the recording layers at a relatively high density when used in optical recording media and irradiated with laser beams with wavelengths around 650 nm. The present invention was made based on the creation of the novel organic dye compounds and the discovery of their industrially useful characteristics.